1. Field of the Invention
The present invention relates to an on-demand multi-nozzle ink jet head using layered piezoelectric elements (hereinafter referred to as "piezoelectric stack"), and more particularly, to an adhesive material for bonding the piezoelectric stack to a diaphragm.
2. Description of the Prior Art
Currently, the most widely used ink jet printing method is the on-demand method, in which ink is ejected only when a print signal is received. Examples of this on-demand method well known in the art include the thermal jet method, which heats the ink directly with a heater and uses air bubbles generated on the surface of the heater to pressurize the ink in a pressurizing chamber, and the piezoelectric method, in which a piezoelectric stack is driven to decrease the internal volume of the pressurizing chamber.
In the piezoelectric method, it is particularly important to establish a satisfactory bond between the piezoelectric stack and the diaphragm to ensure that displacements of the piezoelectric stack are transferred efficiently to the pressurizing chamber. As described in Japanese Patent Application Laid-Open Publication (Kokai) No. SHO-62-73952, for example, mechanical transformations of a piezoelectric stack can be efficiently transferred via the diaphragm to the ink in the pressurizing chamber if the piezoelectric stack is bonded to the diaphragm using an adhesive material with a Shore hardness of 40 or greater on the D scale. Using nozzles with this construction, it is possible to provide a very reliable ink jet head.
An example of a conventional ink jet head is given in FIG. 1. As shown therein, a substrate 19 formed with a groove that corresponds to a channel is joined with a diaphragm 20 to form an ink channel 21 and a nozzle 22. A metal plate 24 is fixed to the diaphragm 20 via an electrically conductive adhesive material 23. On the metal plate 24, are disposed, in order, another layer of the adhesive material 23, a piezoelectric stack 25, a thin film electrode 26, and a solder bump 27.
In order to eject ink during a printing process, a power source 29 applies a drive voltage V0 to the piezoelectric stack 25 via a switch 28. The mechanical transformation generated in the piezoelectric stack 25 and metal plate 24 is transferred in order via the adhesive material 23 and diaphragm 20 to ink 30, thereby forcing the ink 30 outward. This process causes a droplet 31 of ink to be ejected from the nozzle 22 in the ink ejection direction 32. After ink ejection, the piezoelectric stack 25 returns to its original shape, and ink is supplied through the ink supply opening 33 in the ink supply direction 34 to replace the amount of ink that was ejected.
An ink jet head with the construction described above is generally called a Kyser type ink jet head and described in, for example, U.S. Pat. No. 3,946,398. However, if the piezoelectric stack and diaphragm are bonded together using a soft adhesive material, this material will absorb the vibrations of the piezoelectric stack, preventing ink ejection from the nozzle.
This type of ink jet head is typically configured with a plurality of piezoelectric stacks arranged in alignment with one another on a substrate. A plurality of nozzles are formed corresponding to respective ones of the piezoelectric stacks individually. Ink is ejected from the nozzles by displacing the corresponding piezoelectric stacks in the d.sub.33 direction. If the piezoelectric stacks are bonded to the diaphragm with an adhesive material having a Shore hardness of 40 or greater on the D scale, and neighboring nozzles eject ink droplets at the same time, both corresponding channels are mutually affected by one another and are unable to sufficiently cancel the meniscus vibrations. This effect reduces the speed of the ejected droplets, causing irregularity in the ejection properties, or results in a secondary droplet being ejected after the first. Both of these problems invite a decline in printing quality.